Sur une affection parasitaire de la glande digestive de l'huitre plate, Ostrea Edulis Linne

During the summer of 1968, an abnormal mortality rate struck the flat oyster, O. edulis, raised in the upstream part of the Aber Wrach, a ria of Brittany's north coast. During the autumn, the phenomenon expanded steadily and progressively reached the various sectors of the estuary up to its mouth. Losses often reached and exceeded half the stock of oysters of all ages that were held there. The molluscs that survived, whether they were left where they were or transferred to other oyster-farming areas, had a deficient look characterised by a lack of growth, paleness of the digestive gland and an often very marked weight loss. On the weakest ones, the parasite flagellate Hexamita sp. was frequently found. In January 1969, changes similar to those noticed in C. angulata were observed on the gills of some oysters. A culture of the environment using thioglycolate attempted by GRAS allowed him to find forms similar to those of L. marina (= Dermocystidium marinum) (GRAS, 1969; GRAS and HERRBACH, 1971; ALDERMAN and GRAS, 1969). As for FRANC and ARVY (1970), studying a batch of oysters from the same place, they discovered on the coat of one single individual in that batch a colony of an organism closely related, if not similar, to T. polymorpha, described by them as responsible for C. angulata's gill disease. Nevertheless, the authors do not establish any link between the recorded mortality rate and the presence of these organisms. Moreover, the frequency of branchial changes remained low, and the causes of the mortality rate were researched at the beginning, either in a "contaminated area" attributed to silt removal work performed during the summer of 1968 in Aber Wrach's port, or in during a change of physical-chemical conditions that occurred during reproduction that was taking place late. It was in June 1969 that we were able to start the histological study of the samples previously recorded, and of those that were harvested at a later date in the Aber Wrach itself or in the Aber Benoît, which is next to it, or in Brest's harbour where batches of oysters had been transported. Furthermore, when new mortalities occurred in 1970 in the Abers and in other French sectors, notably in Arcachon and in the northwest of Spain, we had the opportunity to study important material and make useful comparisons. This work allowed us to detect the presence of parasites in the digestive gland of sick oysters, peculiar spherical organisms and incidentally ciliates (Fig. 1).